Shadow mask sidewall tunnel junction for quantum computing

ABSTRACT

A technique relates to forming a sidewall tunnel junction. A first conducting layer is formed using a first shadow mask evaporation. A second conducting layer is formed on a portion of the first conducting layer, where the second conducting layer is formed using a second shadow mask evaporation. An oxide layer is formed on the first conducting layer and the second conducting layer. A third conducting layer is formed on part of the oxide layer, such that the sidewall tunnel junction is positioned between the first conducting layer and the third conducting layer.

DOMESTIC PRIORITY

This application is a continuation of U.S. application Ser. No.15/616,193, titled “SHADOW MASK SIDEWALL TUNNEL JUNCTION FOR QUANTUMCOMPUTING”, filed Jun. 7, 2017, the contents of which are incorporatedby reference herein in its entirety.

BACKGROUND

The present invention generally relates to superconducting devices. Morespecifically, the present invention relates to shadow mask sidewalltunnel junctions for quantum computing applications.

Evaporation is a common method of thin-film deposition. The sourcematerial is evaporated in a vacuum. The vacuum allows vapor particles totravel directly to the target object (substrate) where they condenseback to a solid state. Evaporation is used in microfabrication. Duringevaporation, a hot source material evaporates and then condenses on thesubstrate. Evaporation takes place in a vacuum, i.e., vapors other thanthe source material are almost entirely removed before the processbegins. In a high vacuum (with a long mean free path), evaporatedparticles can travel directly to the deposition target without collidingwith the background gas. At a typical pressure of 4-10 Pascals (Pa), a0.4 nanometer particle has a mean free path of 60 meters. Evaporatedatoms that collide with foreign particles can react with them. Forexample, if aluminum is deposited in the presence of oxygen, it willform aluminum oxide. Evaporated materials deposit non-uniformly if thesubstrate has a rough surface (as integrated circuits often do). Becausethe evaporated material attacks the substrate mostly from a singledirection, protruding features block the evaporated material from someareas. This phenomenon is called “shadowing” or “step coverage.”

A common technique for the fabrication of Josephson junctions involvesdouble-angle shadow evaporation of aluminum through an offset mask,wherein the tunnel barrier is formed by the diffusive oxidation of thealuminum base layer. Shadow evaporation has been the most successfulfabrication approach to date for making long-lived, high-coherencesuperconducting quantum bits (or qubits).

The Niemeyer-Dolan technique, also called the Dolan technique or theshadow evaporation technique, is a thin-film lithographic method tocreate nanometer-sized overlapping structures. This technique uses anevaporation mask that is suspended above the substrate. The evaporationmask can be formed from two layers of resist. Depending on theevaporation angle, the shadow image of the mask is projected ontodifferent positions on the substrate. By carefully choosing the anglefor each material to be deposited, adjacent openings in the mask can beprojected on the same spot, creating an overlay of two thin films with awell-defined geometry

New shadow mask evaporation techniques are needed to form tunneljunctions, such as Josephson junctions for superconducting quantumcomputing applications. In particular, new techniques are sought whichcan reduce variability of fabrication.

SUMMARY

Embodiments of the present invention are directed to a method of forminga sidewall tunnel junction. A non-limiting example of the methodincludes forming a first conducting layer using a first shadow maskevaporation, and forming a second conducting layer on a portion of thefirst conducting layer, where the second conducting layer is formedusing a second shadow mask evaporation. The method includes forming anoxide layer on the first conducting layer and the second conductinglayer and forming a third conducting layer on part of the oxide layer,such that the sidewall tunnel junction is positioned between the firstconducting layer and the third conducting layer.

Embodiments of the invention are directed to a method of forming asidewall tunnel junction. A non-limiting example of the method includesforming a non-metal layer having a height dimension greater than a widthdimension, forming a first conducting layer on a portion of thenon-metal layer, where the first conducting layer is formed using afirst shadow mask evaporation, forming an oxide layer on the firstconducting layer, and forming a second conducting layer on part of theoxide layer, such that the sidewall tunnel junction is positionedbetween the first conducting layer and the second conducting layer.

Embodiments of the present invention are directed to a method of forminga sidewall tunnel junction. A non-limiting example of the methodincludes forming a first conducting layer using a first shadow maskevaporation, where the first conducting layer has a width dimensiongreater than a height dimension, and forming a second conducting layeron top of a portion of the first conducting layer, where the secondconducting layer is formed using a second shadow mask evaporation. Themethod includes forming an oxide layer on the first conducting layer andthe second conducting layer, where a part of the first conducting layeris underneath the second conducting layer, and forming a thirdconducting layer on a region of the oxide layer, such that the sidewalltunnel junction is positioned between the second conducting layer andthe third conducting layer. The sidewall tunnel junction is alsopositioned between the part of the first conducting layer and the thirdconducting layer.

Embodiments of the present invention are directed to method of forming asidewall tunnel junction. A non-limiting example of the method includesforming a first conducting layer using a first shadow mask evaporation,where the first conducting layer has a height dimension greater than awidth dimension, forming an oxide layer on the first conducting layer,and forming a second conducting layer on the oxide layer covering a topportion and a side portion of the first conducting layer, such that thesidewall tunnel junction is positioned between the second conductinglayer and the top and side portions of the first conducting layer. Thesecond conducting layer is formed using a second shadow maskevaporation.

Embodiments of the invention are directed to a tunnel junction device. Anon-limiting example of the device includes a first conducting layerhaving a height dimension greater than a width dimension, an oxide layerformed on the first conducting layer, and a second conducting layer onthe oxide layer covering a side portion of the first conducting layer,such that the oxide layer forms a sidewall tunnel junction between thesecond conducting layer and the side portion of the first conductinglayer.

Additional technical features and benefits are realized through thetechniques of the present invention. Embodiments and aspects of theinvention are described in detail herein and are considered a part ofthe claimed subject matter. For a better understanding, refer to thedetailed description and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The specifics of the exclusive rights described herein are particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features and advantages ofthe embodiments of the invention are apparent from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1 depicts a top view of fabricating a Josephson tunnel junctiondevice according to embodiments of the present invention;

FIG. 2 depicts a top view of fabricating the Josephson tunnel junctiondevice according to embodiments of the present invention;

FIG. 3 depicts a cross-sectional view of FIG. 2 according to embodimentsof the present invention;

FIG. 4 depicts a top view of fabricating the Josephson tunnel junctiondevice according to embodiments of the present invention;

FIG. 5 depicts a cross-sectional view of FIG. 4 according to embodimentsof the present invention;

FIG. 6 depicts a top view of fabricating the Josephson tunnel junctiondevice according to embodiments of the present invention;

FIG. 7 depicts a cross-sectional view of FIG. 6 according to embodimentsof the present invention;

FIG. 8 depicts a top view of fabricating the Josephson tunnel junctiondevice according to embodiments of the present invention;

FIG. 9 depicts a cross-sectional view of FIG. 8 according to embodimentsof the present invention;

FIG. 10 depicts a top view of fabricating another Josephson tunneljunction device according to embodiments of the present invention;

FIG. 11 depicts a cross-sectional view of FIG. 10 according toembodiments of the present invention;

FIG. 12 depicts a top view of fabricating the Josephson tunnel junctiondevice according to embodiments of the present invention;

FIG. 13 depicts a cross-sectional view of FIG. 12 according toembodiments of the present invention;

FIG. 14 depicts a top view of fabricating the Josephson tunnel junctiondevice according to embodiments of the present invention;

FIG. 15 depicts a cross-sectional view of FIG. 14 according toembodiments of the present invention;

FIG. 16 depicts a top view of fabricating yet another Josephson tunneljunction device according to embodiments of the present invention;

FIG. 17 depicts a cross-sectional view of FIG. 16 according toembodiments of the present invention;

FIG. 18 depicts a top view of fabricating the Josephson tunnel junctiondevice according to embodiments of the present invention;

FIG. 19 depicts a cross-sectional view of FIG. 18 according toembodiments of the present invention;

FIG. 20 depicts a top view of fabricating the Josephson tunnel junctiondevice according to embodiments of the present invention;

FIG. 21 depicts a cross-sectional view of FIG. 20 according toembodiments of the present invention;

FIG. 22 depicts a top view of fabricating another Josephson tunneljunction device according to embodiments of the present invention;

FIG. 23 depicts a cross-sectional view of FIG. 22 according toembodiments of the present invention;

FIG. 24 depicts a top view of fabricating the Josephson tunnel junctiondevice according to embodiments of the present invention;

FIG. 25 depicts a cross-sectional view of FIG. 24 according toembodiments of the present invention;

FIG. 26 depicts a flow chart of a method of forming a sidewall tunneljunction of an out-of-plane Josephson junction device according toembodiments of the present invention;

FIG. 27 depicts a flow chart of a method of forming a sidewall tunneljunction of an out-of-plane Josephson junction device according toembodiments of the present invention;

FIG. 28 depicts a flow chart of a method of forming a sidewall tunneljunction of an out-of-plane Josephson junction device according toembodiments of the present invention;

FIG. 29 depicts a flow chart of a method of forming a sidewall tunneljunction of an out-of-plane Josephson junction device according toembodiments of the present invention;

FIG. 30 is a top view depicting concepts according to embodiments of thepresent invention;

FIG. 31 is a cross-sectional view of FIG. 30 to illustrate conceptsaccording to embodiments of the present invention; and

FIG. 32 is a cross-sectional view of FIG. 30 to illustrate conceptsaccording to embodiments of the present invention; and

FIG. 33 is a cross-sectional view of FIG. 30 to illustrate conceptsaccording to embodiments of the present invention.

The diagrams depicted herein are illustrative. There can be manyvariations to the diagram or the operations described therein withoutdeparting from the spirit of the invention. For instance, the actionscan be performed in a differing order or actions can be added, deletedor modified. Also, the term “coupled” and variations thereof describeshaving a communications path between two elements and does not imply adirect connection between the elements with no interveningelements/connections between them. All of these variations areconsidered a part of the specification.

In the accompanying figures and following detailed description of thedisclosed embodiments, the various elements illustrated in the figuresare provided with two or three digit reference numbers. With minorexceptions, the leftmost digit(s) of each reference number correspond tothe figure in which its element is first illustrated.

DETAILED DESCRIPTION

For the sake of brevity, conventional techniques related tosemiconductor device and integrated circuit (IC) fabrication may or maynot be described in detail herein. Moreover, the various tasks andprocess steps described herein can be incorporated into a morecomprehensive procedure or process having additional steps orfunctionality not described in detail herein. In particular, varioussteps in the manufacture of semiconductor devices andsemiconductor-based ICs are well known and so, in the interest ofbrevity, many conventional steps will only be mentioned briefly hereinor will be omitted entirely without providing the well-known processdetails.

Turning now to an overview of technologies that are more specificallyrelevant to aspects of the invention, technologically-relevant tunneljunctions for application in quantum computing are made withsuperconductors and have dimensions of about 100 nanometers on a side.The two main types of tunnel junctions utilized by researchers in thefield of quantum computing are obtained by the Dolan Bridge or Manhattantechniques. Both produce in-plane tunnel junction geometries. Anin-plane tunnel junction has its greatest area in-plane, and if theplane is defined by the surface of the substrate (e.g., which containsthe orthogonal x and y axes), then the greatest area belonging to thetunnel junction is contained in the plane containing the x and y axes.The smallest area would be defined by a direction not contained in thesurface of the substrate (e.g., the perpendicular z-axis, or atilted/angled axis not parallel to the substrate) and another orthogonaldirection contained in the plane (e.g., x-axis, y-axis or a linearcombination of the two), as compared to the in-plane area (contained inthe plane of the x and y axes). Two orthogonal directions are at rightangles to each other. A tilted/angled direction is non-orthogonal withthe plane of the substrate (but may be orthogonal with a directioncontained in the plane). The plane containing the x and y axes may alsodesignate a plane that is parallel (e.g., not intersecting) with thesurface of the substrate. The plane containing the x and y axes can alsodesignate a plane that is in close proximity to the surface of thesubstrate but not intersecting it. A tunnel junction made by the DolanBridge technique is referred to as a Dolan junction, while a tunneljunction made by the Manhattan technique is referred to as a Manhattanjunction. A Manhattan junction can be fabricated utilizing a patternknown as a Manhattan crossing during lithography, named as such becauseit has intersecting streets and avenues at right angles.

Turning now to an overview of the aspects of the invention, one or moreembodiments of the invention address the above-described shortcomings ofthe prior art by providing a Josephson tunnel junction with out-of-planegeometry. The out-of-plane geometry can include sidewall geometry or fingeometry. In the out-of-plane Josephson junction device, the Josephsonjunction area is perpendicular to or tilted/angled with respect to theplane of the substrate, and the Josephson junction area is dominated bythe thickness of a layer such as the deposited film or a pre-existingfin. As used in embodiments of the present invention, shadow mask orangle evaporation are fabrication techniques that can be utilized inmaking high quality superconducting qubits. A Josephson junction is atype of tunnel junction, which consists of superconducting metal oneither side of a weak link, such as an oxide layer (known as a tunnelbarrier), a short section of non-superconducting metal, or a physicalconstriction in a superconductor. The superconducting metal (instead ofregular metal) makes a Josephson junction a special type of tunneljunction. Finally, a superconducting qubit is a special case of a qubitand is made using one or more Josephson junctions. Therefore, theJosephson junction is the required component of a superconducting qubit.Other qubits exist but are not made of Josephson junctions.

More specifically, the above-described aspects of the invention addressthe shortcomings of the prior art by providing a structure that has anout-of-plane Josephson tunnel junction and is made by shadow maskevaporation. Critical current (I_(c)) of a Josephson junction is bestpredicted by the area of the tunnel junction (for a given oxidationcondition). This out-of-plane Josephson junction (or area of theout-of-plane Josephson junction) is controlled by the lithographicdimension and film thickness. The uncertainty in the out-of-planeJosephson junction area is less dependent on the lithographic dimensionsthan in-plane techniques. Therefore, an immediate advantage ofout-of-plane Josephson junctions is that the imprecision in the area ofthe junction is a combination of the imprecision of one lithographicdimension and the imprecision of the thickness of the layer whichprovides the out-of-plane junction. If the imprecision of the thicknessis smaller than the imprecision of one lithographic dimension, this typeof junction should result in lower overall imprecision than for thosein-plane junctions that depend on the imprecision of two lithographicdimensions. One other beneficial aspect of embodiments of the presentinvention relies on the out-of-plane overlap contributing significantlyto the area of the Josephson tunnel junction. Additionally, theJosephson junction device having this out-of-plane Josephson junction isreproducible for very small junctions.

Turning now to a more detailed description of aspects of the presentinvention, FIGS. 1-9 depict fabrication of a Josephson tunnel junctiondevice 100 according to embodiments of the present invention. FIG. 1depicts a top view of fabricating a Josephson tunnel junction device 100according to embodiments of the present invention. A resist layer 106 isdeposited on a substrate 108. The resist layer 106 can be a single orbilayer resist, can include one or more underlayers, such as ananti-reflective coating (ARC), a planarizing layer, or hardmaskmaterials, or can be another stack including a resist, as understood byone skilled in the art. The resist layer 106 is patterned to have aManhattan crossing of trenches 102 and 104 that expose the substrate108. In the Manhattan technique, trench 102 is generally referred to asan avenue and trench 104 is generally referred to as a street. Thesubstrate 108 can be a wafer on a wafer stage, and the wafer stage holdsand moves the wafer substrate during the fabrication as understood byone skilled in the art.

Non-limiting examples of suitable materials for the substrate 108include Si (silicon), strained Si, SiC (silicon carbide), Ge(germanium), SiGe (silicon germanium), SiGeC (silicon-germanium-carbon),Si alloys, Ge alloys, III-V materials (e.g., GaAs (gallium arsenide),InAs (indium arsenide), InP (indium phosphide), or aluminum arsenide(AlAs)), II-VI materials (e.g., CdSe (cadmium selenide), CdS (cadmiumsulfide), CdTe (cadmium telluride), ZnO (zinc oxide), ZnSe (zincselenide), ZnS (zinc sulfide), ZnTe (zinc telluride)), sapphire, orquartz, or any combination thereof. Other non-limiting examples ofsemiconductor materials include III-V materials, for example, indiumphosphide (InP), gallium arsenide (GaAs), aluminum arsenide (AlAs), orany combination thereof. The III-V materials can include at least one“III element,” such as aluminum (Al), boron (B), gallium (Ga), indium(In), and at least one “V element,” such as nitrogen (N), phosphorous(P), arsenic (As), antimony (Sb).

The pattern of trenches 102 and 104 in the resist layer 106 can beperformed by lithographic patterning and followed by development of theresist layer 106. In one case, the patterning of the trenches 102 and104 can be by photolithographic patterning that patterns the resist 106on the substrate 108, and the development process can be, for example,TMAH developer. Additional developers are generally known in the art.The pattern of trenches 102 and 104 in the resist layer 106 canalternatively be performed by lithographic patterning and followed by anetching. In one case, the patterning of the trenches 102 and 104 can beby photolithographic patterning that patterns the resist 106 on thesubstrate 108, and the etching process can be, for example, a reactiveion etching process that removes exposed portions of the resist 106 inorder to form desired patterns discussed herein.

Additionally, it should be noted that junctions in embodiments of theinvention can be made with a single step of lithography. By not havingmultiple steps of lithography, embodiments of the present invention donot need to remove the resist or perform lift off in between evaporationsteps, and then spin resist again and expose another lithographicpattern. Rather, embodiments of the present invention are specificallydesigned to be used with a single patterning step and multipleevaporations/oxidations done without breaking vacuum in the sameevaporator.

A photoresist is a light-sensitive material. A positive resist is a typeof photoresist in which the portion of the photoresist that is exposedto light becomes soluble to the photoresist developer. The unexposedportion of the photoresist remains insoluble to the photoresistdeveloper. On the other hand, a negative photoresist is a type ofphotoresist in which the portion of the photoresist that is exposed tolight becomes insoluble to the photoresist developer. The unexposedportion of the photoresist is dissolved by the photoresist developer.

Additionally, embodiments of the present invention can utilizeelectron-beam lithography (often abbreviated as e-beam lithography)which is the practice of scanning a focused beam of electrons to drawcustom shapes (i.e., exposing) on a surface covered with anelectron-sensitive film called an electron-beam (or e-beam) resist. Theelectron beam changes the solubility of the electron-beam resist,enabling selective removal of either the exposed or non-exposed regionsof the resist by immersing it in a solvent (i.e., developing). Thepurpose, as with photolithography, is to create very small structures inthe resist that can subsequently be transferred to the substratematerial, often by etching or deposition. Non-limiting examples ofsuitable electron-beam resists include Poly(methyl methacrylate) (PMMA),which is a type of positive electron-beam resist, and Hydrogensilsesquioxane (HSQ), which is a type of negative electron-beam resist.Analogous to photoresist, a positive electron-beam resist is a type ofresist in which the portion of the resist that is exposed to theelectron beam (as opposed to light) becomes soluble to the electron-beamresist developer. The unexposed portion of the electron-beam resistremains insoluble to the electron-beam resist developer. On the otherhand, a negative electron-beam resist is a type of resist in which theportion of the resist that is exposed to the electron beam (as opposedto light) becomes insoluble to the electron-beam resist developer. Theunexposed portion of the electron-beam resist is dissolved by theelectron-beam resist developer. Other methods for performing e-beamlithography, for example but not limited to using lift off resist (LOR)or electron-beam resist bilayers, are understood by one skilled in theart.

FIG. 2 depicts a top view of fabricating the Josephson tunnel junctiondevice 100 according to embodiments of the present invention. FIG. 2illustrates the first shadow evaporation (evaporation #1) along they-axis direction as shown. The first shadow evaporation is configured toform a tall but narrow superconducting film 202 with height H1A (shownin FIG. 3) and width WfinA. The superconducting film 202 is formed inthe trench 102 and on a sidewall 204 of the resist 106 in the trench104. The sidewall 204 of superconducting film 202 runs/extends along thex-axis direction as shown (e.g., has a greater length dimension in thex-axis than dimensions in the y-axis or z-axis) in trench 104 so as tobe perpendicular to the superconducting film 202 (running in they-axis), but these films are not physically connected to each other (forexample, FIGS. 30-33 illustrate views where the sidewall deposit iscollected at the resist and does not connect to the bottom film).Referring to FIG. 2, the thickness of the resist 106 (in the z-axisperpendicular to the page) is designed as tall enough given a tilt angleθ1A relative to the plane of the substrate 108, that superconductingfilm 202 is not connected to sidewall film 204.

For explanation purposes and not to obscure the figures, no deposit ofmaterial such as material from the first, second, third evaporationswill be shown on top of the resist layer 106 or on top of sidewalllayers 204, 404, 604, 1604, 1804, 1904, 2204, or 2404 (in Josephsonjunction devices 100, 1000, 1600, and 2200). Additionally, it should beunderstood that any material on top of the resist layer 106 willeventually be lifted off when the resist layer 106 is removed exceptwhere noted otherwise. Likewise, it should be understood that anymaterial attached only to the sidewall of the resist layer 106, asopposed to being attached to the sidewall and one other surface (such asthe substrate 108 or another evaporated film which is anchored on thesurface), will eventually be lifted off when the resist layer 106 isremoved except where noted otherwise.

It should be understood that lift off is a way to finish the device.Lift off can be done by using a solvent, such as Dow® Microposit™Remover 1165 or acetone, to remove the resist layer at the end, alongwith any materials that are attached to the resist but nothing else. Oneskilled in the art understands how to perform lift off.

FIG. 3 is a cross-sectional view of FIG. 2 according to embodiments ofthe present invention. The cross-section is taken along the dashed linein FIG. 2. The height H1A of the superconducting film 202 in the z-axiscan range from about 30 nanometers (nm) to 300 nm. The width WfinA ofthe superconducting film 202 in the x-axis can range from about 20 nm to200 nm. The first shadow evaporation (#1) is into the page in FIG. 3thereby forming the superconducting film 202. The first shadowevaporation can be a performed at angle θ1A measured fromsubstrate/wafer 108 (or wafer stage holding the substrate 108). Forexample, the source of the evaporation (evaporator) typically evaporatesinto the substrate at a 90° angle (at right angle with the substrate108) relative to the plane of substrate 108. For generating a tilt, theevaporation is performed at a smaller angle to the plane of substrate108 (or wafer stage holding the substrate 108). In some embodiments ofthe present invention, the evaporation tilt angle θ1A used to form thefirst superconducting film 202 during the first evaporation can rangefrom about 30° to 80°.

FIG. 4 depicts a top view of fabricating the Josephson tunnel junctiondevice 100 according to embodiments of the present invention. FIG. 4illustrates rotating the wafer stage 90° (about its center) to performthe second shadow evaporation (e.g., evaporation #2). One skilled in theart should understand that the center is defined by a point in themiddle of a plane, and usually, these wafer stages will be round or atleast flat. Embodiments of the invention are referring to this flatstage about the center of the circle (or a point on the plane).

The second shadow evaporation (evaporation #2) is from the left and isconfigured to form a thin superconducting film 402 with thickness t1A(shown in FIG. 5). The superconducting film 402 is formed in the trench104 and on a sidewall 404 of the resist 106 in the trench 102. Thesidewall 404 formed of superconducting film 402 runs/extends along they-axis direction as shown (e.g., has a greater length dimension in they-axis than dimensions in the x-axis or z-axis) in trench 104 to beperpendicular to the superconducting film 402 (running in the x-axis),but these films are not physically connected to each other (for example,FIGS. 30-33 illustrate views where the sidewall deposit is collected atthe resist and does not connect to the bottom film). The second shadowevaporation creates a gap 406 on the back side of the firstsuperconducting film 202 because the gap 406 is in the (evaporation)shadow of the first superconducting film 202 relative to the secondevaporation angle θ2A relative to the plane of the substrate 108. To theright of gap 406, the shadowing of the second evaporation creates apatch 405 of superconducting film 202.

FIG. 5 depicts a cross-sectional view of FIG. 4 according to embodimentsof the present invention. The cross-section is taken along the dashedline in FIG. 4. The second shadow evaporation (#2) is configured withlow tilt angle θ2A from the substrate 108. In some embodiments of thepresent invention, the second evaporation angle θ2A used to form thesecond superconducting film 402 during the second evaporation can rangefrom about 20° to 80°.

The thickness t1A is greater than or at least approximately equal towidth WfinA of the second superconducting film 402, in order to resultin a continuous second film 402 interrupted only by gap 406. Thethickness t1A is the length of the wedge hypotenuse of the secondsuperconducting film 402 that extends over the first superconductingfilm 202 and is equal to the nominal thickness of material deposited astypically monitored in the evaporation tool. As noted above, sidewall404 does not participate in the device, as it is removed later by liftoff. The sidewall 404 is only attached to the resist 106, and thesidewall 404 receives additional deposition and oxidation but does notparticipate in the device. The thickness t1A can range from about 20 nmto 100 nm. The height H2A of the second superconducting film 402 (andpatch 405) in the z-axis is related to the thickness t1A by thegeometric expression H2A=t1A×sin(θ2A), where angle θ2A is the tilt ofthe second shadow evaporation from the substrate 108, and H2A can rangefrom about 10 nm to 100 nm. The width WfinA of the superconducting film202 in the x-axis can range from about 20 nm to 200 nm.

FIG. 6 depicts a top view of fabricating the Josephson tunnel junctiondevice 100 according to embodiments of the present invention. FIG. 6shows that an oxidation layer 602 is grown on top of firstsuperconducting film 202 and second superconducting film 402. Forexample, the superconducting metal of the first and second films 202 and402 can be oxidized to form the oxidation layer 602. The oxide istypically grown by oxidizing the existing superconductor metal, such asAl, without breaking vacuum. This is done by introducing oxygen gas intothe evaporation chamber or an attached oxidation chamber that the samplecan be transferred to and from without vacuum break at this step.Alternatively, an oxide may be deposited instead of grown. FIG. 7depicts a cross-sectional view of FIG. 6 according to embodiments of thepresent invention. FIG. 7 illustrates that the first superconductingfilm 202 and second superconducting film 402 have the oxidation layer602 on top. Sidewall films 204 and 404 are also oxidized but they do notparticipate in the geometry of the final device because they are onlyattached to the resist layer 106 and will be removed by a lift off stepat a later stage in the fabrication.

FIG. 8 depicts a top view of fabricating the Josephson tunnel junctiondevice 100 according to embodiments of the present invention. Afteroxidation, the wafer stage (holding the substrate 108) is rotated 180°(about its center). Then, the third shadow evaporation (evaporation #3)is performed with the tilt angle θ3A relative to the plane of thesubstrate 108 (which can be the same as evaporation #2) from the rightto form a third superconducting film 802. The third shadow evaporationcreates a gap 806 on the back side of the first superconducting film 202because the gap 806 is in the (evaporation) shadow of the firstsuperconducting film 202, the second superconducting film 402, and theoxidation layer 602 relative to the third evaporation angle θ3A. Thethird shadow evaporation creates a patch 805 of third superconductingfilm 802 on the opposite side of the gap 806. A sidewall film 604 isalso formed on the resist layer 106 which is removed by a lift off stepat a later stage in the fabrication.

FIG. 9 depicts a cross-sectional view of FIG. 8 according to embodimentsof the present invention. The third tilt angle θ3A equals (or is aboutthe same as) the second tile angle θ2A. The thickness of the thirdsuperconducting film 802 is thickness t2A. The thickness t2A is thelength of the wedge hypotenuse of a portion of third superconductingfilm 802 above the first and second films 202 and 402 and above theoxidation layer 602. The third shadow evaporation is performed such thatthe thickness t2A>t1A. The thickness t2A can range from about 50 nm to200 nm. The height H3A of the third superconducting film 802 satisfiesthe relation H3A=t2A×sin(θ3A), where θ3A is the tilt of third shadowevaporation (i.e., angle of the source evaporation) from the substrate108. The Josephson tunnel junction device 100 has been formed in FIGS. 8and 9. The tunnel junction has an electrode made by the combination ofsuperconducting films 402 and 202, superconducting film 802, and theportion of oxide layer 602 sandwiched between the two electrodes. TheJosephson tunnel junction device 100 has an out-of-plane tunneljunction. The out-of-plane tunnel junction includes a vertical tunneljunction 910 and an angled tunnel junction 912, as it relates to theflow of electrical current depicted by the electrical current flow arrowin FIG. 8. The vertical tunnel junction 910 is a portion of the oxidelayer 602 that has a greater dimension in the z-axis (verticaldirection) than in the x-axis (width direction). The angled tunneljunction 912 has a length greater than its thickness. A Josephson tunneljunction is formed by two superconducting films (i.e., superconductingelectrodes) sandwiching the oxide layer 602. Also, other Josephsonjunctions at the far left and far right are formed, but these otherJosephson junctions do not contribute to and/or affect the electricalcurrent flow (i.e., critical current that flows at superconductingtemperatures).

Particularly, junction area=street width×(H1A+t1A). The street width isthe width of the trench 104 (i.e., street) along the y-axis minus thewidth along the y-axis of the sidewall deposit 204 (aside some edgeimprecision). The street width can range from about 50 nm to 350 nm.

In some embodiments, the thickness t1A (of second superconducting film402) or t2A (of third superconducting film 802) is the wedge hypotenuse(or length) because wedge hypotenuse (or length) coincides with thethickness given by the film thickness monitor that is used in theevaporator. The source evaporator is the device that contains thematerial (i.e., first, second, and third superconducting films) to bedeposited by evaporation as understood by one skilled in the art.

As noted above, the Josephson junction area (of the vertical tunneljunction 910) is mainly defined by the oxide layer 602 in between the(right) side of the first superconducting film 202 and the (left) sideof the third superconducting film 802. However, a smaller part of theJosephson junction area (i.e., the angled tunnel junction 912) can bedefined by the oxide layer 602 in between the wedge hypotenuse (withlength t1A) of second superconducting film 402 (above the firstsuperconducting film 202) and the portion of the third superconductingfilm 802 immediately above the wedge hypotenuse (with length t1A) ofsecond superconducting film 402. An arrow illustrating the electricalcurrent flow is shown in FIG. 8 but not in FIG. 9. In FIG. 9 for thevertical tunnel junction 910, as an illustration from left to right, theelectrical current flows into the left of the second superconductingfilm 402, through the first superconducting film 202, through thevertical tunnel junction (oxide layer 602), and into the thirdsuperconducting film 802. For the angled tunnel junction 912, a smallportion of the electrical current can flow up the first superconductingfilm 202 and/or up the second superconducting film 402, through theangled oxide layer 602, and into the third superconducting film 802.

The first, second, and third superconducting films 202, 402, and 802 caneach be the same superconducting material. In some embodiments of thepresent invention, one or more of the first, second, and thirdsuperconducting films 202, 402, and 802 can be different from oneanother.

There are various options in the fabrication of the Josephson tunneljunction device 100. The first, second, and third shadow evaporationscan each use the same tilt angle, such that the tilt angle θ1A=tiltangle θ2A=tilt angle θ3A. As another option, the tilt angle θ3A for thethird shadow evaporation is larger than the tilt angle θ2A for thesecond evaporation. Additionally, in order to overcome patch 405 on theright, height H3A=t2A×sin(θ3A) of third superconducting film 802 isgreater or equal to height H2A=t1A×sin(θ2A) of second superconductingfilm 402 and patch 405. Otherwise, patch 405 could create a shadow inthird superconducting film 802 and a gap separating thirdsuperconducting film 802 to the left of the patch 405 to thirdsuperconducting film 802 deposited on the back (i.e., on top) of patch405. To ensure that no new gap is created by the shadow of patch 405,the height H3A can be at least twice (at least two times) the heightH2A.

In the figures, it is noted that the wafer stage holding the substrate108 can be rotated about its center. However, the figures maintain thex-axis as being horizontal for ease of understanding.

FIGS. 10-15 depict fabrication of a Josephson tunnel junction device1000 according to embodiments of the present invention, as discussedfurther below. FIG. 10 depicts a top view of fabricating a Josephsontunnel junction 100 according to embodiments of the present invention.FIG. 11 depicts a cross-section view of FIG. 10 according to embodimentsof the present invention, taken along the dashed line in FIG. 10.

A step edge 1002 is formed on the substrate 108. The step edge 1002 istall with height H1B but narrow with width WfinB. The step edge 1002 canbe formed using lithography and etching. In some embodiments of thepresent invention, the step edge 1002 can be etched in the substrate108. In some embodiments of the present invention, the step edge 1002can be a layer that is deposited on the substrate 108 and then etchedinto the desired shape. The step edge 1002 is made of a material that isnot superconducting (and not a regular metal). In some embodiments ofthe present invention, the material of the step edge 1002 can includesilicon. In some embodiments of the present invention, the step edge1002 can be an insulating material. The height H1B of the step edge 1002in the z-axis can range from about 30 nm to 300 nm. The width WfinB ofthe step edge 1002 in the x-axis can range from about 20 nm to 200 nm.

A resist layer 106 is formed on top of the step edge 1002 and thesubstrate 108. The resist layer 106 can be patterned/formed to have thetrench 104 (i.e., street). However, no trench 102 is needed in thisexample (i.e., avenue). The trench 104 is to be used for shadowevaporation.

FIG. 12 depicts a top view of fabricating the Josephson tunnel junction1000 according to embodiments of the present invention. FIG. 13 depictsa cross-section view of FIG. 12 according to embodiments of the presentinvention, taken along the dashed line in FIG. 12.

The wafer stage (holding the substrate 108) is rotated 90° about itscenter. The first shadow evaporation is performed to deposit the firstsuperconducting film 1202 in the trench 104. The first shadowevaporation is from the left with a tilt angle θ1B relative to thesubstrate 108 to form a thin first superconducting film 1202 withthickness OB. The thickness of the first superconducting film 1202 isthe length of the wedge hypotenuse of a portion of first superconductingfilm 1202 extending above the step edge 1002.

The thickness t1B can range from about 20 nm to 100 nm. The thicknesst1B is greater than or at least approximately equal to WfinB. The heightH2B=t1B×sin(θ1B) denotes the height of the first superconducting film1202. The height H1B of the step edge 1002 causes a gap 1206 in thefirst superconducting film 1202 during the first shadow evaporation attilt angle θ1B and a patch 1205 (of the first superconducting film 1202)on the right. The tilt angle θ1B can range from about 20° to 80°.

FIG. 14 depicts a top view of fabricating the Josephson tunnel junction1000 according to embodiments of the present invention. FIG. 15 depictsa cross-section view of FIG. 14 according to embodiments of the presentinvention, taken along the dashed line in FIG. 14. An oxide layer 1404is formed on top of the first superconducting film 1202 just asdiscussed in FIGS. 1-9. The oxide layer 1404 can be the same oxidematerial as the oxide layer 602 in some embodiments of the presentinvention. In other embodiments of the present invention, the oxidelayer 1404 can be a different oxide material than the oxide layer 602.

After oxidation (or before), the wafer stage is rotated 180° about itscenter. Then, the second shadow evaporation (#2) is performed with tiltangle θ2B to deposit a second superconducting film 1402 in the trench104. The second shadow evaporation can be performed with (about) thesame tilt (angle θ2B=angle θ1B) to deposit a thicker layer of the secondsuperconducting film 1402 than the first superconducting film 1202. Thesecond shadow evaporation results in a patch 1405 of the secondsuperconducting film 1402. The tilt angle θ2B can range from about 20°to 80°. The thickness t2B of the second superconducting film is thewedge hypotenuse (length). The Josephson junction device 1000 has thecondition in which the thickness t2B>t1B. The thickness t2B can rangefrom about 50 nm to 200 nm. The height H3B=t2B×sin(θ2B) denotes theheight of the second superconducting film 1402. Alternatively torotating the stage by 180° and tilting the stage by an angle θ2B, onecan also not rotate the stage and use a tilt angle of 180°-θ2B andachieve the same evaporation (#2) result.

The Josephson tunnel junction device 1000 has an out-of-plane tunneljunction. The out-of-plane tunnel junction includes an angled tunneljunction 1512, as it relates to the flow of electrical current depictedby the electrical current flow arrow in FIG. 14. The angled tunneljunction 1512 has a length greater than its thickness, and the angledtunnel junction 1512 is angled downward from left to right in the x-axisin FIG. 15. As utilized for electrical current flow, the Josephsontunnel junction of the oxide layer 1404 is formed between the triangularportion of the first superconducting film 1202 (denoted by length t1B)and the triangular second superconducting film 1402 (denoted by lengtht2B). Part of the angled tunnel junction 1512 is above the step edge1002 and the other part extends above a vertical portion of the firstsuperconducting film 1202.

Particularly, the junction area≈(street width)×(t1B). The street widthis the width of the trench 104 (i.e., street) along the y-axis. Thefirst superconducting film 1202 is in the trench 104, therebyapproximately matching the street width (aside from imprecision in thedeposition and lithography, such as evaporation into an undercut in theresist layer 106). In other words, there is no sidewall narrowing of thefirst superconducting film 1202 resulting from fabrication of theJosephson tunnel junction device 1000 in FIGS. 10-15, corresponding tothe sidewall deposits 204, 404 and 604. The benefit of not havingsidewall narrowing is greater uniformity of the street width across anextended sample, since the shadow from the sidewall deposit can possiblyintroduce additional imprecision to the street width, and it depends onthe thickness of deposited material from the first evaporation asrelated to first film 202 in FIGS. 2-3. The street width can range fromabout 50 nm to 350 nm.

An arrow illustrating the electrical current flow is shown in FIG. 14but not in FIG. 15. As an illustration from left to right, theelectrical current flows into the left of the first superconducting film1202, through the first superconducting film 1202, through the angledtunnel junction 1512 (oxide layer 1404), into the second superconductingfilm 1402 (triangular portion with length t2B), down through the secondsuperconducting film 1402, and out the right side of the secondsuperconducting film 1402.

The first and second superconducting films 1202 and 1402 can each be thesame superconducting material. In some embodiments of the presentinvention, the first and second superconducting films 1202 and 1402 canbe different superconducting materials.

There are various options in the fabrication of the Josephson tunneljunction device 1000. In some embodiments of the present invention, thesecond shadow evaporation (angle θ2B) can use a larger tilt angle thanthe first shadow evaporation (angle θ1B). In order to overcome patch1205 on the right, height H3B=t2B×sin(θ2B) of second superconductingfilm 1402 is greater or equal to height H2B=t1B×sin(θ1B) of firstsuperconducting film 1202 and patch 1205. Otherwise, patch 1205 couldcreate a shadow in second superconducting film 1402 and a new gapseparating second superconducting film 1402 to the left of the patch1205 to second superconducting film 1402 deposited on the back (i.e., ontop) of patch 1205. To ensure that no new gap is created by the shadowof patch 1205, the height H3B can be at least twice (at least two times)the height H2B. Additionally, the step edge 1002 can be made of resist,and the step edge 1002 can be removed (or dissolved, or chemicallyetched away), thereby leaving an open space under the angled tunneljunction 1512 (i.e., without lifting off the material above the stepedge 1002).

FIGS. 16-21 depict fabrication of a Josephson tunnel junction device1600 according to embodiments of the present invention. FIG. 16 depictsa top view of fabricating a Josephson tunnel junction 1600 according toembodiments of the present invention. The resist layer 106 is formed ina pattern on the substrate 108, and the resist layer 106 is patterned tohave a Manhattan crossing of trenches 102 (i.e., avenue) and 104 (i.e.,street) that exposes the substrate 108. In this case, the width of thetrench 104 is wider in the y-axis on the left side of the trench 102than on the right side of the trench 102.

FIG. 16 illustrates the first shadow evaporation (evaporation #1) alongthe y-axis (i.e., in parallel with the y-axis). FIG. 17 is across-sectional view of FIG. 16 according to embodiments of the presentinvention, taken along the dashed line in FIG. 16. The first shadowevaporation (#1) is configured to form a thin first superconducting film1602 with height H1C (shown in FIG. 17) and width WfinC. The firstsuperconducting film 1602 is formed in the trench 102 and on a sidewall1604 of the resist 106 in the trench 104. The sidewall 1604 of the firstsuperconducting film 1602 runs/extends along the x-axis (e.g., has agreater length dimension in the x-axis than dimensions in the y-axis orz-axis) in trench 104 to be perpendicular to the part of the firstsuperconducting film 1602 (running in the y-axis), and it (sidewall1604) does not make contact to the first superconducting film 1602because it is attached to resist layer 106 but not to substrate 108 orto first superconducting film 1602. Sidewall deposit 1604 only depositson the right side of trench 104 and not on the substrate 108 inside theright side of trench 104 because the resist layer 106 creates a shadowover the entire right side of trench 104 over its (narrow) street widthR when the first shadow evaporation (#1) has an angle θ1C measured fromsubstrate/wafer 108. Sidewall deposit 1604 deposits on both the leftside of trench 104 and on the substrate 108 inside the left side oftrench 104 because the resist layer 106 does not create a shadow overthe left side of trench 104 over its (wide) street width L when thefirst shadow evaporation (#1) has the same angle θ1C measured fromsubstrate/wafer 108 (for example, FIG. 33 provides further understandingof this type of evaporation).

The first shadow evaporation (#1) is into the page in FIG. 16 therebyforming the first superconducting film 1602. The first shadowevaporation can be performed at angle θ1C measured from substrate/wafer108 (or wafer stage holding the substrate 108). The source of theevaporation (evaporator) typically evaporates into the substrate at a90° angle (at right angle with the substrate 108) relative to the planeof substrate 108. For generating a tilt, the evaporation is performed ata smaller angle to the plane of substrate 108 (or wafer stage holdingthe substrate 108). In some embodiments of the present invention, theevaporation tilt angle θ1C used to form the first superconducting film1602 during the first shadow evaporation can range from about 20° to80°.

The thickness t1C is the thickness of the first superconducting film1602 in the evaporation monitor. The thickness t1C of the firstsuperconducting film 1602 can range from about 20 nm to 100 nm. Theheight H1C of the first superconducting film 1602 is H1C=t1C×sin(θ1C).The height H1C of the first superconducting film 1602 in the z-axis canrange from about 10 nm to 100 nm. It is noted that the height H1C is afraction of t1C and will be always smaller, by construction. Thethickness t1C (relative to H1C) is shown for explanation purposes and toease understanding. The width WfinC (in the trench 102) of the firstsuperconducting film 1602 along the x-axis can range from about 20 nm to200 nm.

FIG. 18 depicts a top view of fabricating a Josephson tunnel junction1600 according to embodiments of the present invention. FIG. 19 is across-sectional view of FIG. 18 according to embodiments of the presentinvention, taken along the dashed line in FIG. 18. The second shadowevaporation (evaporation #2) is again along the y-axis (i.e., inparallel with the y-axis), which is like the first shadow evaporation.The second shadow evaporation (#1) is configured to form a tall butnarrow second superconducting film 1802 with height H2C (shown in FIG.19) and width WfinC. The second superconducting film 1802 is formed inthe trench 102, and on a sidewall 1804 of the previously formed sidewall1604 (which in turn is on the sidewall of the resist 106). The sidewall1804 of the second superconducting film 1802 runs/extends along thex-axis (e.g., has a greater length dimension in the x-axis thandimensions in the y-axis or z-axis) in trench 104 to be perpendicular tothe part of the second superconducting film 1802 (running in they-axis), and not connected to it (because the shadow deposition ends onprevious sidewall 1604).

The second shadow evaporation (#2) is also into the page in FIG. 18thereby forming the second superconducting film 1802. The second shadowevaporation can be a performed at angle θ2C measured fromsubstrate/wafer 108 (or wafer stage holding the substrate 108). Again,the source of the evaporation (evaporator) typically evaporates into thesubstrate at a 90° angle (at right angle with the substrate 108)relative to the plane of substrate 108. For generating a tilt, theevaporation is performed at a smaller angle to the plane of substrate108 (or wafer stage holding the substrate 108). In some embodiments ofthe present invention, the evaporation tilt angle θ2C used to form thesecond superconducting film 1802 during the second shadow evaporationcan range from about 10° to 60°. The angle θ2C has a very shallow tiltangle from the plane of the substrate compared to angle θ1C (i.e., angleθ1C>angle θ2C). For simplicity, the relationship between the heights ofthe first and second superconducting films 1602 and 1802 can be heightH2C>>height H1C.

FIG. 20 depicts a top view of fabricating a Josephson tunnel junction1600 according to embodiments of the present invention. FIG. 21 is across-sectional view of FIG. 20 according to embodiments of the presentinvention, taken along the dashed line in FIG. 20. An oxide layer 1922is formed on top of the first superconducting film 1602 and secondsuperconducting film 1802 (just as discussed in FIGS. 1-15). The oxidelayer 1922 can be the same oxide material as the oxide layer 602 in someembodiments of the present invention. In other embodiments of thepresent invention, the oxide layer 1922 can be a different oxidematerial than the oxide layer 602.

After oxidation (or before), the wafer stage is rotated 90° (about itscenter). Then, the third shadow evaporation (#3) is performed with tiltangle θ3C to deposit a third superconducting film 1902 in the trench104. The third shadow evaporation is performed from the right with atilt angle θ3C to deposit the third superconducting film 1902 andresults in a patch 1905 of third superconducting film 1902 on theopposite side. The tilt angle θ3C can range from about 20° to 80°. Thethickness t2C of the third superconducting film 1902 is the wedgehypotenuse (length). The thickness t2C can range from about 50 nm to 200nm.

The Josephson tunnel junction device 1600 has an out-of-plane tunneljunction. The out-of-plane tunnel junction includes a vertical tunneljunction 2110, as it relates to the flow of electrical current depictedby the electrical current flow arrow in FIG. 20. The vertical tunneljunction 2110 has a height (in the z-axis) greater than its width in thex-axis as depicted in FIG. 21. As utilized for electrical current flow,the Josephson tunnel junction of the oxide layer 1922 is formed between(right side of) the second superconducting film 1802 and (left side of)the third superconducting film 1902. A very small part of the verticaltunnel junction 2110 is formed between the right side of the firstsuperconducting film 1602 and a small left side of the thirdsuperconducting film 1902. Although a small tunnel junction is above thesecond superconducting film 1802, this tunnel junction does notcontribute (or insignificantly adds) to the electrical current flow, aslong as height H2C>>fin width WfinC. It is noted that an in-plane tunneljunction 2114 is formed, but the in-plane tunnel junction 2114 does notcontribute to and/or marginally contributes to the electrical currentflow.

Particularly, the junction area ≈(active street width)×(H1C+H2C+WfinC)as shown in FIGS. 20 and 21. The active street width is the street widthR of the right side of trench 104 (i.e., street) along the y-axis minusthe combined y-axis widths of sidewall deposits 1604 and 1804 on theright side of 104. The active street width can range from about 20 nm to200 nm. It is noted that as long as height H2C>>height H1C, thethickness H1C is irrelevant for junction area in this embodiment and canbe omitted. It is noted that as long as height H2C>>width WfinC, thewidth WfinC is irrelevant for junction area in this embodiment and canbe omitted.

An arrow illustrating the electrical current flow is shown in FIG. 20but not in FIG. 21. As an illustration from left to right, theelectrical current flows into the left of the first superconducting film1602, into the second superconducting film 1802, through the verticaltunnel junction 2110 (vertical oxide layer 1922), into the thirdsuperconducting film 1902, and out the right side of the thirdsuperconducting film 1902.

The first, second, and third superconducting films 1602, 1802, and 1902can each be the same superconducting material. In some embodiments ofthe present invention, one or more of the first, second, and thirdsuperconducting films 1602, 1802, and 1902 can be different from oneanother.

There are various options in the fabrication of the Josephson tunneljunction device 1600. In some embodiments of the present invention, thefirst shadow evaporation (angle θ1C) can be equal to the third shadowevaporation (angle θ3C). It is assumed that the height H2C is much, muchgreater than the height H1C to arrive at the expression for the junctionarea≈(active street width)×(H2C+WfinC).

FIGS. 22-24 depict fabrication of a Josephson tunnel junction device2200 according to embodiments of the present invention. FIG. 22 depictsa top view of fabricating a Josephson tunnel junction 2200 according toembodiments of the present invention. As discussed above, the resistlayer 106 is formed in a pattern on the substrate 108, and the resistlayer 106 is patterned to have a Manhattan crossing of trenches 102 and104 that exposes the substrate 108.

FIG. 22 illustrates the first shadow evaporation (evaporation #1) alongthe y-axis (i.e., in parallel with the y-axis). FIG. 23 is across-sectional view of FIG. 22 according to embodiments of the presentinvention. The cross-section is taken along the dashed line in FIG. 22.The first shadow evaporation (#1) is configured to form a tall butnarrow first superconducting film 2202 with height H1D (shown in FIG.23) and width WfinD. The height H1D can range from about 30 nm to 300nm. The width WfinD can range from about 20 nm to 200 nm. The firstsuperconducting film 2202 is formed in the trench 102 and on a sidewall2204 of the resist 106 in the trench 104. The sidewall 2204 of the firstsuperconducting film 2202 runs/extends along the x-axis (e.g., has agreater length dimension in the x-axis than dimensions in the y-axis orz-axis) in trench 104 to be perpendicular to the part of the firstsuperconducting film 2202 (running in the y-axis), and is not connectedto 2202.

The first shadow evaporation (#1) is into the page in FIG. 23 therebyforming the first superconducting film 2202. The first shadowevaporation can be performed at angle θ1D measured from substrate/wafer108 (or wafer stage holding the substrate 108). The source of theevaporation (evaporator) typically evaporates into the substrate at a90° angle (at right angle with the substrate 108) relative to the planeof substrate 108. For generating a tilt, the evaporation is performed ata smaller angle to the plane of substrate 108 (or wafer stage holdingthe substrate 108). In some embodiments of the present invention, theevaporation tilt angle θ1D used to form the first superconducting film2202 during the first shadow evaporation can range from about 30° to80°.

FIG. 24 depicts a top view of fabricating the Josephson tunnel junctiondevice 2200 according to embodiments of the present invention. FIG. 25depicts a cross-sectional view of FIG. 24 according to embodiments ofthe present invention. The cross-section is taken along the dashed linein FIG. 24. An oxide layer 2422 is formed on top of the firstsuperconducting film 2202 (just as discussed in FIGS. 1-21). The oxidelayer 2422 can be the same oxide material as the oxide layer 602 in someembodiments of the present invention. In other embodiments of thepresent invention, the oxide layer 2422 can be a different oxidematerial than the oxide layer 602.

After oxidation (or before), the wafer stage is rotated 90° (about itscenter). FIG. 24 illustrates rotating the wafer stage 90° (relative tothe evaporation source) to perform the second shadow evaporation(evaporation #2). The second shadow evaporation (#2) is from the rightand is configured to form a second superconducting film 2402 along witha patch 2405 of second superconducting film 2402. The secondsuperconducting film 2402 is formed in the trench 104 and on a sidewall2404 of the resist 106 in the trench 102. The sidewall 2404 formed ofsecond superconducting film 2402 runs/extends along the y-axis (e.g.,has a greater length dimension in the y-axis than dimensions in thex-axis or z-axis) in trench 102 to be perpendicular to the portion ofthe second superconducting film 2402 (running in the y-axis), and it isnot connected to 2402. The second shadow evaporation creates a gap 2406on the back side of the first superconducting film 2202 because the gap2406 is in the (evaporation) shadow of the first superconducting film2202 relative to the second evaporation angle θ2D.

The second superconducting film 2402 has a thickness t1D and a heightH2D. The thickness t1D is greater than or at least approximately equalto width WfinD of the first superconducting film 2202. The thickness t1Dis the length of the wedge hypotenuse of the second superconducting film2402 that extends over the first superconducting film 2202 and over thesidewall 2404. The thickness t1D can range from about 20 nm to 100 nm.The height H2D of the second superconducting film 402 in the z-axis canrange from about 10 nm to 100 nm. The height H2D=t1D×sin(θ2D), whereangle θ2D is the tilt of the second shadow evaporation from thesubstrate 108. In some embodiments of the present invention, theevaporation tilt angle θ2D used to form the second superconducting film2402 during the second shadow evaporation can range from about 20° to80°.

The Josephson tunnel junction device 2200 has an out-of-plane tunneljunction. The out-of-plane tunnel junction includes a vertical tunneljunction 2510, as it relates to the flow of electrical current depictedby the electrical current flow arrow in FIG. 24. The vertical tunneljunction 2510 has a height (in the z-axis) greater than its width in thex-axis as depicted in FIG. 25. As utilized for electrical current flow,the Josephson tunnel junction of the oxide layer 2422 is formed between(right side of) the first superconducting film 2202 and (left side of)the second superconducting film 2402. Although a small in-plane tunneljunction 2514 is above the first superconducting film 2202, this tunneljunction does not contribute (or insignificantly adds) to the electricalcurrent flow.

Particularly, the junction area≈(street width)×(H1D+WfinD). The streetwidth is the width of the trench 104 (i.e., street) along the y-axisminus the sidewall deposit 2204 width along the y-axis. The street widthincludes the first and second superconducting films 2202 and 2402. Thestreet width can range from about 50 nm to 350 nm. It is noted that aslong as height H1D>>width WfinD, the width WfinD is irrelevant forjunction area determination in this embodiment and can be omitted.

An arrow illustrating the electrical current flow is shown in FIG. 24but not in FIG. 25. It is noted that the first superconducting film 2202extends in and out of the page in FIG. 25. As an illustration ofelectrical current flow, the electrical current flows into the firstsuperconducting film 2202 by flowing into the page in the y-axis (inFIG. 25), flows through the vertical tunnel junction 2510 (verticaloxide layer 2422) by flowing in the right direction along the x-axis,into the second superconducting film 2402, and out the right side of thesecond superconducting film 2402.

The first and second superconducting films 2202 and 2402 can each be thesame superconducting material. In some embodiments of the presentinvention, one or more of the first and second superconducting films2202 and 2402 can be different from one another.

There are various options in the fabrication of the Josephson tunneljunction device 2200. In some embodiments of the present invention, thefirst shadow evaporation (angle θ1D) can be equal to the second shadowevaporation (angle θ2D). It is noted that this is a 90° Josephsonjunction, meaning that the current path across the Josephson junctionmakes a 90° turn in the top view (FIG. 24).

FIG. 26 depicts a flow chart 2600 of a method of forming a sidewalltunnel junction of an out-of-plane Josephson junction device 100according to embodiments of the present invention. At block 2602, afirst conducting layer 202 is formed using a first shadow maskevaporation (at angle θ1A). At block 2604, a second conducting layer 402is formed on a portion of the first conducting layer 202, where thesecond conducting layer 402 is formed using a second shadow maskevaporation (at angle θ2A). At block 2606, an oxide layer 602 is formedon the first conducting layer 202 and the second conducting layer 402.At block 2608, a third conducting layer 802 is formed on a region of theoxide layer 602, such that the sidewall tunnel junction (e.g., verticaltunnel junction 910) is positioned between the first conducting layer202 and the third conducting layer 802.

A segment of the sidewall tunnel junction (e.g., angled tunnel junction912) is positioned between the second conducting layer 402 (e.g.,triangular portion of second conducting layer 602 above vertical portionof second conducting layer 602 and above first conducting layer 202 inFIG. 9) and the third conducting layer (e.g., triangular portion ofthird conducting layer 802 above vertical portion of second conductinglayer 602 and above first conducting layer 202 in FIG. 9). The segmentof the sidewall tunnel junction is above the first conducting layer. Thefirst, second, and third conducting layers 202, 402, 802 aresuperconducting materials. The oxide layer 602 is an oxide of the firstand second conducting layers 202, 402.

FIG. 27 depicts a flow chart 2700 of a method of forming a sidewalltunnel junction of an out-of-plane Josephson junction device 1000according to embodiments of the present invention. At block 2702, anon-metal layer 1002 (i.e., step edge) is formed with a height dimension(e.g., in the z-axis) greater than a width dimension (e.g., in thex-axis). At block 2704, a first conducting layer 1202 is formed on aportion of the non-metal layer 1002, where the first conducting layer1202 is formed using a first shadow mask evaporation (e.g., at angleθ1B). At block 2706, an oxide layer 1404 is formed on the firstconducting layer 120. At block 2708, a second conducting layer 1402 isformed on part of the oxide layer 1404, such that the sidewall tunneljunction (e.g., out-of-plane angled tunnel junction 1512) is positionedbetween the first conducting layer 1202 and the second conducting layer1402.

The sidewall tunnel junction (e.g., out-of-plane angled tunnel junction1512) is above the non-metal layer 1002 in FIG. 15. The first conductinglayer 1202 is formed on one side (e.g., the left side in FIGS. 13 and15) of the non-metal layer 1002. The second conducting layer 1402 isformed on another side (e.g., the right side in FIG. 15) of thenon-metal layer opposite the first conducting layer. The first andsecond conducting layers are superconducting materials.

FIG. 28 depicts a flow chart 2800 of a method of forming a sidewalltunnel junction of an out-of-plane Josephson junction device 1600according to embodiments of the present invention. At block 2802, afirst conducting layer 1602 is formed using a first shadow maskevaporation (e.g., at angle θ1C), where the first conducting layer 1602has a width dimension (e.g., in the x-axis) greater than a heightdimension (e.g., in the z-axis). At block 2804, a second conductinglayer 1802 is formed on top of a portion of the first conducting layer1602, where the second conducting layer 1802 is formed using a secondshadow mask evaporation (e.g., at angle θ2C). At block 2806, an oxidelayer 1922 is formed on the first conducting layer 1602 and the secondconducting layer 1802, where a part (e.g., the bottom right portion inFIG. 21) of the first conducting layer 1602 is underneath the secondconducting layer 1802. At block 2808, a third conducting layer 1902 isformed on part of the oxide layer 1922, such that the sidewall tunneljunction (e.g., vertical tunnel junction 2110) is positioned between thesecond conducting layer 1802 and the third conducting layer 1902, wherethe sidewall tunnel junction (e.g., bottom portion of vertical tunneljunction 2110 in FIG. 21) is also positioned between the part of thefirst conducting layer 1602 and (a bottom portion in FIG. 21) the thirdconducting layer 1902.

A top portion of the second conducting layer 1802 is covered by theoxide layer 1922, such that an in-plane tunnel junction 2114 is alsopositioned between the third conducting layer 1902 and the top portionof the second conducting layer 1802. The oxide layer 1922 covers a sideportion (e.g., left side) of the second conducting layer 1802, and theside portion is opposite the sidewall tunnel junction (e.g., oppositethe vertical tunnel junction 2110). The first, second, and thirdconducting layers 1602, 1802, 1902 are superconducting materials.

FIG. 29 depicts a flow chart 2900 of a method of forming a sidewalltunnel junction of an out-of-plane Josephson junction device 2200according to embodiments of the present invention. At block 2902, afirst conducting layer 2202 is formed using a first shadow maskevaporation (at angle θ1D), where the first conducting layer 2202 has aheight dimension (e.g., in the z-axis in FIG. 23) greater than a widthdimension (e.g., in the x-axis in FIG. 23). At block 2904, an oxidelayer 2422 is formed on the first conducting layer 2202. At block 2906,a second conducting layer 2402 on the oxide layer 2422 covering a topportion (to form an in-plane tunnel junction 2514) and a side portion ofthe first conducting layer 2202, such that the sidewall tunnel junction(e.g., the vertical tunnel junction 2510) is positioned between thesecond conducting layer 2402 and the top and side portions of the firstconducting layer 2202. The second conducting layer 2402 is formed by asecond shadow mask evaporation (at angle θ2D).

The oxide layer 2422 covers another side portion of the first conductinglayer 2202, where the another side portion (e.g., left side of the firstconducting layer 2202) being opposite the side portion (e.g., rightside). The second conducting layer 2402 is absent from the oxide layer2422 covering the another side portion (e.g., left side of the firstconducting layer 2202) of the first conducting layer 2202. A portion(e.g., on the left side in FIG. 25) of the second conducting layer 2402is spaced apart from the first conducting layer 2202. Additionally, anembodiment of the invention can be engineered without this “patch”existing. The resist on the left side of the street can be made closerto the avenue, and then this portion lands on the resist and is liftedoff at the end.

The first and second conducting layers 2202, 2402 are superconductingmaterials. The first and second conducting layers 2202, 2402 can be thesame material or different materials. The oxide layer 2422 is an oxideof the first conducting layer 2202.

Examples of superconducting materials (at low temperatures, such asabout 10-100 millikelvin (mK), or about 4 K) include niobium, aluminum,tantalum, etc. For example, the Josephson junctions are made ofsuperconducting material, and their tunnel junctions can be made of athin tunnel barrier, such as an oxide. Any transmission lines (i.e.,wires) connecting the various elements are made of a superconductingmaterial.

FIG. 30 is a top view depicting concepts according to embodiments of thepresent invention. FIG. 31 is a cross-sectional view taken from FIG. 30to illustrate concepts according to embodiments of the presentinvention. FIG. 32 is a cross-sectional view taken from FIG. 30 toillustrate concepts according to embodiments of the present invention.FIG. 33 is a cross-sectional view taken from FIG. 30 to illustrateconcepts according to embodiments of the present invention. FIGS. 30,31, 32 and 33 depict an example of how shadow evaporation can work. Forexample, FIG. 31 illustrates when a shadow makes the evaporation(material) land on the substrate (deposited film), while FIG. 32illustrates when a shadow makes the evaporation (material) land only onthe resist (sidewall deposit and top deposit). Finally, FIG. 33illustrates when a shadow makes the evaporation (material) landpartially on the substrate (deposited film), and partially on the resist(sidewall deposit and top deposit). In FIG. 33, some of the evaporation(material) lands in the region between the top resist layer and thesubstrate. This region can be created using a bottom resist layer withundercut, as understood by one skilled in the art. The evaporation(material) that lands on the top resist layer in FIGS. 31, 32 and 33 isnot shown in the top view of FIG. 30 for simplicity. The sidewalldeposit of FIGS. 32 and 33 lands only on the top resist and not on thesubstrate, and it is not connected to any deposited film that landsdirectly on the substrate.

Various embodiments of the present invention are described herein withreference to the related drawings. Alternative embodiments can bedevised without departing from the scope of this invention. Althoughvarious connections and positional relationships (e.g., over, below,adjacent, etc.) are set forth between elements in the followingdescription and in the drawings, persons skilled in the art willrecognize that many of the positional relationships described herein areorientation-independent when the described functionality is maintainedeven though the orientation is changed. These connections and/orpositional relationships, unless specified otherwise, can be direct orindirect, and the present invention is not intended to be limiting inthis respect. Accordingly, a coupling of entities can refer to either adirect or an indirect coupling, and a positional relationship betweenentities can be a direct or indirect positional relationship. As anexample of an indirect positional relationship, references in thepresent description to forming layer “A” over layer “B” includesituations in which one or more intermediate layers (e.g., layer “C”) isbetween layer “A” and layer “B” as long as the relevant characteristicsand functionalities of layer “A” and layer “B” are not substantiallychanged by the intermediate layer(s).

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

Additionally, the term “exemplary” is used herein to mean “serving as anexample, instance or illustration.” Any embodiment or design describedherein as “exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs. The terms “at least one”and “one or more” are understood to include any integer number greaterthan or equal to one, i.e. one, two, three, four, etc. The terms “aplurality” are understood to include any integer number greater than orequal to two, i.e. two, three, four, five, etc. The term “connection”can include an indirect “connection” and a direct “connection.”

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedcan include a particular feature, structure, or characteristic, butevery embodiment may or may not include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

For purposes of the description hereinafter, the terms “upper,” “lower,”“right,” “left,” “vertical,” “horizontal,” “top,” “bottom,” andderivatives thereof shall relate to the described structures andmethods, as oriented in the drawing figures. The terms “overlying,”“atop,” “on top,” “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements such as an interfacestructure can be present between the first element and the secondelement. The term “direct contact” means that a first element, such as afirst structure, and a second element, such as a second structure, areconnected without any intermediary conducting, insulating orsemiconductor layers at the interface of the two elements.

The phrase “selective to,” such as, for example, “a first elementselective to a second element,” means that the first element can beetched and the second element can act as an etch stop.

The terms “about,” “substantially,” “approximately,” and variationsthereof, are intended to include the degree of error associated withmeasurement of the particular quantity based upon the equipmentavailable at the time of filing the application. For example, “about”can include a range of ±8% or 5%, or 2% of a given value.

As previously noted herein, for the sake of brevity, conventionaltechniques related to semiconductor device and integrated circuit (IC)fabrication may or may not be described in detail herein. By way ofbackground, however, a more general description of the semiconductordevice fabrication processes that can be utilized in implementing one ormore embodiments of the present invention will now be provided. Althoughspecific fabrication operations used in implementing one or moreembodiments of the present invention can be individually known, thedescribed combination of operations and/or resulting structures of thepresent invention are unique. Thus, the unique combination of theoperations described in connection with the fabrication of asemiconductor device according to the present invention utilize avariety of individually known physical and chemical processes performedon a semiconductor (e.g., silicon) substrate, some of which aredescribed in the immediately following paragraphs.

In general, the various processes used to form a micro-chip that will bepackaged into an IC fall into four general categories, namely, filmdeposition, removal/etching, semiconductor doping andpatterning/lithography. Deposition is any process that grows, coats, orotherwise transfers a material onto the wafer. Available technologiesinclude physical vapor deposition (PVD), chemical vapor deposition(CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE)and more recently, atomic layer deposition (ALD) among others.Removal/etching is any process that removes material from the wafer.Examples include etch processes (either wet or dry), andchemical-mechanical planarization (CMP), and the like. Semiconductordoping is the modification of electrical properties by doping, forexample, transistor sources and drains, generally by diffusion and/or byion implantation. These doping processes are followed by furnaceannealing or by rapid thermal annealing (RTA). Annealing serves toactivate the implanted dopants. Films of both conductors (e.g.,poly-silicon, aluminum, copper, etc.) and insulators (e.g., variousforms of silicon dioxide, silicon nitride, etc.) are used to connect andisolate transistors and their components. Selective doping of variousregions of the semiconductor substrate allows the conductivity of thesubstrate to be changed with the application of voltage. By creatingstructures of these various components, millions of transistors can bebuilt and wired together to form the complex circuitry of a modernmicroelectronic device. Semiconductor lithography is the formation ofthree-dimensional relief images or patterns on the semiconductorsubstrate for subsequent transfer of the pattern to the substrate. Insemiconductor lithography, the patterns are formed by a light sensitivepolymer called a photo-resist (and/or alternatively by e-beam resist ine-beam lithography). To build the complex structures that make up atransistor and the many wires that connect the millions of transistorsof a circuit, lithography and etch pattern transfer steps are repeatedmultiple times. Each pattern being printed on the wafer is aligned tothe previously formed patterns and slowly the conductors, insulators andselectively doped regions are built up to form the final device.

The flowchart and block diagrams in the Figures illustrate possibleimplementations of fabrication and/or operation methods according tovarious embodiments of the present invention. Variousfunctions/operations of the method are represented in the flow diagramby blocks. In some alternative implementations, the functions noted inthe blocks can occur out of the order noted in the Figures. For example,two blocks shown in succession can, in fact, be executed substantiallyconcurrently, or the blocks can sometimes be executed in the reverseorder, depending upon the functionality involved.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdescribed herein.

What is claimed is:
 1. A method of forming a sidewall tunnel junctioncomprising: forming a first conducting layer using a first shadow maskevaporation; forming a second conducting layer on a portion of the firstconducting layer, the second conducting layer being formed using asecond shadow mask evaporation; forming an oxide layer on the firstconducting layer and the second conducting layer; and forming a thirdconducting layer on part of the oxide layer, such that the sidewalltunnel junction is positioned between the first conducting layer and thethird conducting layer.
 2. The method of claim 1, wherein a segment ofthe sidewall tunnel junction is positioned between the second conductinglayer and the third conducting layer.
 3. The method of claim 2, whereinthe segment of the sidewall tunnel junction is above the firstconducting layer.
 4. The method of claim 1, wherein the first, second,and third conducting layers are superconducting materials.
 5. The methodof claim 1, wherein the oxide layer is an oxide of the first and secondconducting layers.
 6. A method of forming a sidewall tunnel junctioncomprising: forming a non-metal layer having a height dimension greaterthan a width dimension; forming a first conducting layer on a portion ofthe non-metal layer, the first conducting layer being formed using afirst shadow mask evaporation; forming an oxide layer on the firstconducting layer; and forming a second conducting layer on part of theoxide layer, such that the sidewall tunnel junction is positionedbetween the first conducting layer and the second conducting layer. 7.The method of claim 6, wherein the sidewall tunnel junction is above thenon-metal layer.
 8. The method of claim 6, wherein the first conductinglayer is formed on one side of the non-metal layer.
 9. The method ofclaim 8, wherein the second conducting layer is formed on another sideof the non-metal layer opposite the first conducting layer.
 10. Themethod of claim 6, wherein the first and second conducting layers aresuperconducting materials.
 11. A method of forming a sidewall tunneljunction comprising: forming a first conducting layer using a firstshadow mask evaporation, the first conducting layer having a widthdimension greater than a height dimension; forming a second conductinglayer on top of a portion of the first conducting layer, the secondconducting layer being formed using a second shadow mask evaporation;forming an oxide layer on the first conducting layer and the secondconducting layer, a part of the first conducting layer being underneaththe second conducting layer; and forming a third conducting layer on aregion of the oxide layer, such that the sidewall tunnel junction ispositioned between the second conducting layer and the third conductinglayer, wherein the sidewall tunnel junction is also positioned betweenthe part of the first conducting layer and the third conducting layer.12. The method of claim 11, wherein a top portion of the secondconducting layer is covered by the oxide layer, such that an in-planetunnel junction is positioned between the third conducting layer and thetop portion of the second conducting layer.
 13. The method of claim 11,wherein the oxide layer covers a side portion of the second conductinglayer, the side portion being opposite the sidewall tunnel junction. 14.The method of claim 11, wherein the first, second, and third conductinglayers are superconducting materials.
 15. A method of forming a sidewalltunnel junction comprising: forming a first conducting layer using afirst shadow mask evaporation, the first conducting layer having aheight dimension greater than a width dimension; forming an oxide layeron the first conducting layer; and forming a second conducting layer onthe oxide layer covering a top portion and a side portion of the firstconducting layer, such that the sidewall tunnel junction is positionedbetween the second conducting layer and the top and side portions of thefirst conducting layer, wherein the second conducting layer is formedusing a second shadow mask evaporation.
 16. The method of claim 15,wherein the oxide layer covers another side portion of the firstconducting layer, the another side portion being opposite the sideportion.
 17. The method of claim 15, wherein the second conducting layeris absent from the oxide layer covering the another side portion of thefirst conducting layer.
 18. The method of claim 15, wherein the firstand second conducting layers are made of the same material.
 19. Themethod of claim 15, wherein the first and second conducting layers aresuperconducting materials.
 20. The method of claim 15, wherein the oxidelayer is an oxide of the first conducting layer.